The advent of novel electron emissive materials to low pressure and high pressure type lamps has lead to substantial increases in lamp operating lifetimes.
One example is the use of barium peroxide and tantalate metal to produce a fused-porous electron emissive body having a density gradient structure as disclosed in the U.S. Pat. No. 3,798,492 issued to R. Menelly and assigned to the assignee of the instant invention. Menelly teaches that when the electron emissive material is given the porosity and the bulk density gradient as described therein, the electron emission readily occurs at lower electrode operational temperatures than with the standard alkaline earth carbonate mix.
Another example of novel emissive materials is the use of alkaline earth tantalate emissive coatings as disclosed within the following U.S. applications: Ser. No. 487,409, filed July 10, 1974, entitled "Emissive Coating for Electrodes"; and Ser. No. 487,082, filed July 10, 1974, entitled "Electron Emissive Coatings". Kern teaches the use of barium tantalate having a high barium oxide to tantalum oxide ratio and barium calcium tantalate where the barium and calcium content are approximately equivalent and where the barium-calcium oxide content is high relative to the tantalum oxide content to produce large quantities of emissive coatings for fluorescent lamp electrodes. However the use of fused density gradient structures and tantalate compounds adversely effect some of the lamp operating properties. One of the lamp properties particularly involved is the incidence of formation of end discoloratin which is the dark deposit that occurs in the vicinity of the lamp operating electrodes at both ends of the lamp after a few hundred hours of lamp operation. As described in the aforementioned applications these deposits are believed to comprise compounds formed by the reaction of barium metal with mercury and oxides of mercury.
Since the barium-tantalum and barium-calcium-tantalum compounds are rendered electron emissive by controlled heating in the absence of oxygen it is felt that excess barium may be produced in the heat treatment process. The unusual early occurrence of end discoloration with these tantalate compounds may quite possibly be due to the presence of the excess barium and calcium on the surface of the compound. Since the elemental barium and calcium vapor pressure is quite high relative to the vapor pressure of the barium-calcium tantalate a series of experiments were designed in order to determine whether the excess elemental barium and calcium could be removed by thermal evaporation without affecting the electron emissive properties of the overall tantalate compound.